Liquid level measuring devices have been known for many years. Their purpose is to locate the level of a liquid, or to indicate the amount of liquid remaining in a container.
On many occasions, monitoring the amount of a liquid in a container is required. However, direct observation of the liquid level is not always possible or practical. Measurement of the liquid in such containers as pressurized cylinders, sealed containers, cryogenic flasks, and opaque vessels is often difficult. Such measurements are even more troublesome when the liquid within the container is corrosive or potentially toxic or flammable.
Sight glasses and weight scales are some examples of liquid level measuring devices which are commonly employed. Both of these devices suffer from a number of disadvantages. Sight glasses are expensive, and they can crack and break easily. On such occasions where the container is placed outdoors, ultraviolet light can cause the glass to haze. Moreover, sight glasses may crack in cold weather, especially when used with water tanks, and are subject as well to salt scale deposits and the concomitant loss of visibility of the level of liquid in the sight glass. Weight scales are also expensive, and in many instances, measurements provided by weight scales are inexact.
A simple, economical external liquid level gauge which permits a direct reading of the level of a flowable material has been provided by the present inventor in Canadian Patent No. 1,177,281 issued on Nov. 6, 1984. The liquid level measuring device taught therein employs one thermochromatic material which is coated onto a base layer. The base layer is magnetically mounted to the outside surface of the outside wall of the container, and thus the external liquid level gauge can be repeatedly removed and replaced or relocated when necessary.
The theory is that the rate of heat transfer is different between a mass of flowable material and the void volume above it such that for any container with a modest heat conducting capability, the container wall experiences a temperature gradient which is most pronounced at the interface of the contents with the void volume above the contents, and of course below that interface. That is to say, the rate of heat transfer through the wall of a container will be greater where there is a mass of flowable material located in the container than where there is a void volume above the flowable material. In other words, the rate of heat transfer through the container wall changes most abruptly at the level of the interface, and below. Thus, with the use of a thermochromatic material, a vivid color change occurring at the interface, and below, will permit an observer to obtain a direct reading of the level of the flowable material within a container by discerning where the interface is located.
Several other prior art thermochromatic external liquid level gauges are now described. They include GILMOUR U.S. Pat. No. 3,696,675 issued Oct. 10, 1972, which teaches an external liquid level gauge adapted to be permanently affixed to the outside wall of a container for determining the liquid-gas interface within the container. The external liquid level gauge described therein consists of a uniform thermochromatic liquid crystalline material which coats the entire base layer of the gauge such that it is at right angles to the liquid-gas interface. The uniform thermochromatic material covers the entire temperature range to which the container is subjected within an overall range of −20° C. to 250° C. Depending upon the thermochromatic material selected, color changes over a gradient from violet to red can occur in a range as small as 2° C. to one as broad as 150° C. Since the temperature differential across the liquid-gas interface is generally small, on the order of less than 2° C., the change in color is slight across the interface. This is particularly the case when the container is placed outdoors and a large temperature range needs to be covered. As a result, it is difficult to visually locate the liquid-gas interface.
In U.S. Pat. No. 5,323,652 issued Jun. 28, 1994 to PARKER, the inventor teaches a thermochromic level indicator for determining the level of a material inside a container. The thermochromic level indicator includes at least two thermochromic materials of different opacities and transition temperature. Prior to the attachment of the thermochromic level indicator to the outside surface of the outside wall of the container, the thermochromic materials are applied to a transparent film by silk screening, other printing and coating methods, or methods which employ the use of microencapsulated thermochromic materials. The thermochromic level indicator may be permanently adhered to the container wall or it may be adhered to a magnetic strip which can be temporarily affixed to the container wall.
In another U.S. Pat. No. 5,707,590, issued Jan. 13, 1998, the inventor THOMAS et al. has provided a detergent container with a thermochromatic level indicator. In one embodiment of the invention, the thermochromatic substance is added to the container's plastic material during the molding process. In another embodiment of the invention, the level indicator or strip comprises a base material, such as Mylar, which is coated or imbedded with a thermochromatic substance by such methods as painting, stripping, or screen printing.
In yet another U.S. Pat. No. 6,260,414 issued Jul. 17, 2001 to BROWN et al., the inventors teach a cholesteric liquid crystal fluid level indicator that determines the level of a cooled liquid, in particular beer, in a closed, opaque keg, and that indicates whether the beer in the keg is at its ideal temperature for consumption. Specifically, the cholesteric liquid crystal fluid level indicator as taught by BROWN et al. has a liquid crystal composition that produces a color change between a temperature range of 30° F. to 50° F. The inventors therein emphasize that this range is particularly crucial as it is consistent with the temperature of beer so that the level of beer may be readily determined by observing the color change of the cholesteric liquid crystal fluid level indicator. In addition, the cholesteric liquid crystal fluid level indicator produces a predetermined color when the beer is at its ideal temperature for consumption. The liquid crystal composition used in the cholesteric liquid crystal fluid level indicator taught by BROWN et al. is a cholesteric liquid crystal composition or cholesteric/chiral nematic liquid crystal mixture that exhibits at least one color, but preferably three, in a predetermined cooled temperature range. The cholesteric liquid crystal fluid level indicator operates such that when the closed refrigerated environment experiences a slight and sudden change in temperature, such as when the refrigerator door is opened or the compressor turns off, then there will be a specific color display on the indicator from which an observer can determine whether or not the beer within the keg inside the refrigerator is at a suitable temperature for consumption.
It is important to note that the thermochromatic materials employed in the level gauges of all of the foregoing prior art are various forms of cholesteric liquid crystal compositions. As is known to those skilled in the art, cholesteric liquid crystal thermochromics are toxic and unfortunately very difficult to work with as they require highly specialized printing and handling techniques. Typically, silk-screen printing is required for the application of the cholesteric liquid crystal thermochromics onto a desired substrate. Due to the relatively small particle size of the cholesteric liquid crystal thermochromics, fine screen mesh, and thinner laydown are desirable. Such fine screen printing process is slow and tedious. Further, it increases the manufacturing costs of the level gauge, making it expensive and uneconomical to the consumers. Still further, the type of substrates on which the cholesteric liquid crystal thermochromics may be applied are limited. Moreover, typically any gauge which employs liquid crystal thermochromics must also employ an extra backing layer—usually black—on which the liquid crystal thermochromics are placed.
Another disadvantage of employing the use of cholesteric liquid crystal composition is that cholesteric liquid crystal composition typically has a limited shelf life under normal conditions. Thus, over a long period of time, the performance of the level gauges which are manufactured with such cholesteric liquid crystal compositions may deteriorate. Since the level gauges are applied onto containers which are typically placed outdoors and are subjected to harsh environmental conditions, the life span of a cholesteric liquid crystal composition level gauge may even have a further shortened life span. It has been reported that under long periods of UV light, extremely hot temperature and cold temperatures, aggressive spraying of solvents and other fluidic materials onto the level gauge, and excessive washings, significant deterioration of the performance of the cholesteric liquid crystal compositions is shown. Since accuracy of the level gauge is highly desirable in the determination of the level of the interface between a flowable material and the void volume above it within a container, there is a need to provide a level indicator which is manufactured from a material having a long life span and at the same time can withstand harsh environmental conditions.
Moreover, the level gauges of the foregoing prior art are useful for indicating the amount of materials remaining inside a container, but they are not specifically useful as overfill indicators. Indeed, in many instances, only an approximation of the level of materials inside the container is provided. Since the thermochromatic materials present in the prior art level gauges are not in direct contact with the outside surface of the outside wall of the container, the chromatic response of these thermochromatic materials may be delayed. Furthermore, these prior art level gauges may not provide a prominent color change at the level of the interface, and thus a reading of the level of materials may be inexact.
However, under certain circumstances, it is critical to determine the precise level of the materials inside a container, such as in the case of liquefied propane in pressurized cylinder. In warm weather conditions particularly, hydrostatic pressure exerted by liquefied propane inside the pressurized cylinder may cause the cylinder to explode if the cylinder is overfilled. Thus, in order to prevent undesirable gas venting from overfilled cylinders, the United States National Fire Protection Association (NFPA) has recently mandated a safe-fill level of propane in pressurized cylinders to be at a level which is 80% of the volume of the propane cylinders such that a 20% volume head space is maintained when the pressurized cylinders are full of propane. Due to the new NFPA regulations, float valves are being applied to cylinders so as to prevent overfill. The float valve closes when the propane level reaches a volume of 80% of the container.
Although float valves provide a method of preventing overfilling, they are a costly solution. In order for a float valve to be installed in a cylinder, the structure of the cylinder needs to be altered. While some existing cylinders on the market may be retrofitted with float valves, many other existing cylinders must be discarded, and new cylinders with pre-installed float valves need to be manufactured in order to comply with the new regulation. Furthermore, since a float valve is a mechanical device, it is subject to mechanical failure over a period of time.
As is discussed immediately above, installing float valves to cylinders is a solution to prevent propane overfill. However, these float valves may only be compatible to cylinders used in the United States and Canada, and may not be compatible with cylinders used overseas, in such places as Europe, Asia or South America where propane gas is a commonly used cooking fuel. Moreover, many regions outside of North America may not yet have the same or similar regulations as mandated by NFPA, but insurance companies, local municipalities, and the like, may require overfill indicators in order for certain coverage or licensing regulations to be effective.
In light of the foregoing, there is a need to provide a level indicator which employs a thermochromic composition which is economical, non-toxic and easy to handle, and at the same time provides a distinct and responsive color change when the level indicator is in operation. Furthermore, there is a need to provide a level indicator which can be adapted to any container for determining the level of the materials inside the container, and to detect overfill. Still further, there is a need to provide a level indicator which can be easily installed to a container without having to alter the structure of the container, and without the use of tools during the installation process. Moreover, there is a need to provide a level indicator which can be provided to the user at a relatively low cost.
Quite surprisingly, the present inventor has unexpectedly discovered that by using leucodye inks in the thermochromatic layer, a level indicator may be obtained which obviate the disadvantages of using cholesteric liquid crystals which are used in existing level indicators. Since most leucodye inks are formulated with water, they are virtually odor free, and easier to handle. Furthermore, all of the leucodye inks are non-toxic upon drying, which makes them easy to handle and safe to apply to different types of containers. The costs of leucodye inks compound to liquid crystal thermochromics may be at least several orders of magnitude lower, for the same area coverage. Still further, the leucodye inks can be applied with a number of different printing processes. At the same time, the present inventor is able to provide a level gauge which replicates the optical properties of the cholesteric liquid crystal compositions. The leucodye inks exhibit vivid color changes with only slight changes in temperature.